Technical Abstract:
The application of marker assisted selection for quantitative traits is most advantageous when the heritability is low, the traits are sex-limited and the size of the QTL effect is large. QTL for testis size, plasma FSH in boars, and body composition (backfat) have been identified near the centromere on the X chromosome in a Meishan-White Composite resource population which fit these criteria. Sperm production in boars is correlated with testis size and plasma FSH concentrations and is an economically important trait in an industry that has quickly expanded its use of artificial insemination. Production and reproductive traits are genetically correlated, and selection for reproductive traits is usually counteractive to performance traits. Since thyroid function affects Sertoli cell development and adult testis size in rodents, thyroxine-binding globulin (TBG) is a positional candidate gene for testis size. Recently we discovered a polymorphism in exon 2 of the porcine TBG gene that results in the amino acid change of the consensus histidine to an asparagine. This SNP resides in the ligand-binding domain of the mature polypeptide and the Meishan allele is the conserved allele found in human, bovine, sheep and rodent TBG. Mutations in this region of human TBG result in decreased heat stability and affinity for ligand. Functional studies indicate altered binding characteristics of the TBG isoforms. These allelic variants are being evaluated in subsequent generations of the resource population for effects on testis size, thyroid function, and body composition.